Osteocalcin-controlled dissolution-reprecipitation of calcium phosphate under biomimetic conditions

We have examined the influence of osteocalcin, one of the 10 most abundant proteins of the human body, on hydroxyapatite (HAP, Ca-10(PO4)(6)(OH)(2)) f ormation. Different functions in biomineralization are attributed to the bo ne-specific protein osteocalcin because of its Ca2+ binding including HAP b inding properties and its capability to inhibit HAP precipitation. To study nucleation and crystal growth, a model system with osteocalcin-controlled dissolution-reprecipitation of brushite (DCPD, CaHPO4. 2H(2)O) to HAP has b een investigated. After DCPD crystals were grown from aqueous solution, the y were exposed to an osteocalcin-containing buffer solution of pH 7.4. Thin apatite-like crystals with hexagonal symmetry grew on the (010) brushite p lanes. The apatite (0001) planes were fully covered with osteocalcin molecu les. Thus, osteocalcin has been found to regulate HAP formation in two diff erent ways: (i) it accelerates nucleation, and (ii) it acts as a specific i nhibitor of the apatite (0001) plane, supressing crystal growth perpendicul ar to this plane. A stress-induced growth model was developed illustrating HAP growth along the brushite -HAP interface considering compressions in th e protein-covered HAP crystals.